linux-kernel - Re: [PATCHv2 3/5] mm/rmap: mlock large folios in try_to_unmap

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Message-ID: <z3yzhd3c7qw4ig4ekh3mq665zws5aafab7lgqpvytzg5wxv363@ojmp7qdsl5vc>
Date: Mon, 22 Sep 2025 13:16:14 -0700
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Kiryl Shutsemau <kirill@...temov.name>
Cc: Andrew Morton <akpm@...ux-foundation.org>, 
	David Hildenbrand <david@...hat.com>, Hugh Dickins <hughd@...gle.com>, 
	Matthew Wilcox <willy@...radead.org>, Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, 
	"Liam R. Howlett" <Liam.Howlett@...cle.com>, Vlastimil Babka <vbabka@...e.cz>, 
	Mike Rapoport <rppt@...nel.org>, Suren Baghdasaryan <surenb@...gle.com>, 
	Michal Hocko <mhocko@...e.com>, Rik van Riel <riel@...riel.com>, 
	Harry Yoo <harry.yoo@...cle.com>, Johannes Weiner <hannes@...xchg.org>, 
	Baolin Wang <baolin.wang@...ux.alibaba.com>, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCHv2 3/5] mm/rmap: mlock large folios in try_to_unmap_one()

On Mon, Sep 22, 2025 at 10:51:35AM +0100, Kiryl Shutsemau wrote:
> On Fri, Sep 19, 2025 at 02:27:40PM -0700, Shakeel Butt wrote:
> > On Fri, Sep 19, 2025 at 01:40:34PM +0100, Kiryl Shutsemau wrote:
> > > From: Kiryl Shutsemau <kas@...nel.org>
> > > 
> > > Currently, try_to_unmap_once() only tries to mlock small folios.
> > > 
> > > Use logic similar to folio_referenced_one() to mlock large folios:
> > > only do this for fully mapped folios and under page table lock that
> > > protects all page table entries.
> > > 
> > > Signed-off-by: Kiryl Shutsemau <kas@...nel.org>
> > > ---
> > >  mm/rmap.c | 23 ++++++++++++++++++++---
> > >  1 file changed, 20 insertions(+), 3 deletions(-)
> > > 
> > > diff --git a/mm/rmap.c b/mm/rmap.c
> > > index 3d0235f332de..482e6504fa88 100644
> > > --- a/mm/rmap.c
> > > +++ b/mm/rmap.c
> > > @@ -1870,6 +1870,7 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
> > >  	unsigned long nr_pages = 1, end_addr;
> > >  	unsigned long pfn;
> > >  	unsigned long hsz = 0;
> > > +	int ptes = 0;
> > >  
> > >  	/*
> > >  	 * When racing against e.g. zap_pte_range() on another cpu,
> > > @@ -1910,10 +1911,26 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
> > >  		 */
> > >  		if (!(flags & TTU_IGNORE_MLOCK) &&
> > >  		    (vma->vm_flags & VM_LOCKED)) {
> > > +			ptes++;
> > > +			ret = false;
> > > +
> > > +			/* Only mlock fully mapped pages */
> > > +			if (pvmw.pte && ptes != pvmw.nr_pages)
> > > +				continue;
> > > +
> > > +			/*
> > > +			 * All PTEs must be protected by page table lock in
> > > +			 * order to mlock the page.
> > > +			 *
> > > +			 * If page table boundary has been cross, current ptl
> > > +			 * only protect part of ptes.
> > > +			 */
> > > +			if (pvmw.flags & PVMW_PGTABLE_CROSSSED)
> > > +				goto walk_done;
> > 
> > Should it be goto walk_abort?
> 
> I already have to set ret to false above to make it work for partially
> mapped large folios. So walk_done is enough here.

Indeed and I missed that. What do you think about adding a comment when
setting ret to false? Everywhere else we are jumping to abort for
scenarios which needs to break the rmap walk loop but here we need to
keep counting the ptes for mlock handling. Anyways it's just a nit.

Reviewed-by: Shakeel Butt <shakeel.butt@...ux.dev>